Honeycomb filter

ABSTRACT

A honeycomb filter, including:
         a honeycomb structure,   wherein the honeycomb structure further includes a platinum group element-containing catalyst layer and a trapping layer,   the platinum group element-containing catalyst layer is disposed at least on an entire surface of a side of an inner surface of the partition walls surrounding either one of the inflow cells or the outflow cells,   the trapping layer does not contain a platinum group element, and   the trapping layer is disposed in a range of at least up to 50% with respect to an overall length of the honeycomb structure starting from the outflow end face and is not disposed in a range of at least up to 40% with respect to the overall length starting from the inflow end face, in an extending direction of the cells of the honeycomb structure.

The present application is an application based on JP-2018-066814 filed on Mar. 30, 2018 with Japan Patent Office, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a honeycomb filter. More particularly, the present invention relates to a honeycomb filter which is excellent in trapping performance for trapping particulate matter contained in exhaust gas and excellent in purification performance for purifying harmful components contained in the exhaust gas.

Description of the Related Art

In recent years, regulations on removal of particulate matter contained in exhaust gas emitted from a gasoline engine are getting stricter around the world, and a honeycomb filter having a honeycomb structure has been used as a filter for removing particulate matter. Hereinafter, the particulate matter may be referred to as “PM”. The PM is an abbreviation for “particulate matter”.

Examples of the honeycomb filter can include a honeycomb filter which includes a honeycomb structure having porous partition walls defining a plurality of cells and plugging portions plugging an end portion of any one of the cells. In such a honeycomb filter, each of the porous partition walls is structured to serve as a filter for removing the PM. Specifically, the PM-containing exhaust gas is introduced from an inflow end face of the honeycomb filter, the PM is trapped by the porous partition walls to be filtered, and then the purified exhaust gas is emitted from an outflow end face of the honeycomb filter. By doing so, the PM in the exhaust gas can be removed.

For the purpose of improving the purification performance of such a honeycomb filter, loading a catalyst for purifying the exhaust gas onto the porous partition wall is performed (see Patent Document 1, for example). Examples of the catalyst for purifying the exhaust gas can include a platinum group element-containing catalyst constituted by an exhaust gas purifying catalyst containing a platinum group element. Hereinafter, the platinum group element-containing catalyst may be referred to as a “PGM catalyst”. “PGM” is an abbreviation for “platinum group metal”. The PGM includes ruthenium, rhodium, palladium, osmium, iridium, and platinum.

[Patent Document 1] JP-A-2015-066536

SUMMARY OF THE INVENTION

In the honeycomb filter for the gasoline engine, it has been studied to use the honeycomb structure in which a porosity of partition wall is high in order to suppress an increase in pressure loss. When the PGM catalyst is loaded onto the partition walls of such a honeycomb structure, the PGM catalyst is loaded so as to be filled in the pores of the porous partition walls.

Conventionally, the honeycomb filter using the honeycomb structure having the high porosity has a problem in that the trapping performance for trapping the PM deteriorates when the PGM catalyst is loaded. The following reasons are conceivable as a cause of the deterioration in the trapping performance. When the PGM catalyst is loaded onto the partition walls in which a porosity is high, the PGM catalyst is filled in order starting from pores with a relatively smaller pore diameter among the pores of the partition walls. For this reason, in the partition walls onto which a predetermined amount of PGM catalyst is loaded, the pores with a relatively smaller pore diameter are closed by the PGM catalyst and the pores with a relatively larger pore diameter remain. Hereinafter, the pores with a relatively smaller pore diameter are referred to as “small pore”, and the pores with a relatively larger pore diameter are referred to as “large pore”. In the honeycomb structure in which a porosity is high, when the small pores of the partition walls are preferentially closed by the PGM catalyst, the ratio of the large pores of the partition walls is increased and the flow of exhaust gas permeating the partition walls concentrates on the large pores. That is, the exhaust gas becomes difficult to flow in the small pores which contribute to the improvement of the trapping performance, and as a result the trapping performance of the honeycomb filter deteriorates.

The present invention has been made in view of such problems of the prior art. According to the present invention, there is provided a honeycomb filter which is excellent in trapping performance for trapping PM contained in exhaust gas and excellent in purification performance for purifying harmful components contained in the exhaust gas.

According to the present invention, there is provided a honeycomb filter as shown below.

[1] A honeycomb filter including: a honeycomb structure which has porous partition walls disposed so as to surround a plurality of cells as through channels of fluid extending from an inflow end face to an outflow end face; and

plugging portions disposed to plug either one of end portions of each of the cells on the side of the inflow end face or the side of the outflow end face,

wherein the cells in which the plugging portions are arranged in the end portion on the side of the outflow end face and which are opened on the side of the inflow end face are defined as inflow cells,

the cells in which the plugging portions are arranged in the end portion on the side of the inflow end face and which are opened on the side of the outflow end face are defined as outflow cells,

the honeycomb structure further includes a platinum group element-containing catalyst layer constituted by an exhaust gas purifying catalyst containing a platinum group element and a trapping layer which traps particulate matter in exhaust gas,

the platinum group element-containing catalyst layer is disposed at least on an entire surface of a side of an inner surface of the partition walls surrounding either one of the inflow cells or the outflow cells,

the platinum group element-containing catalyst layer is present only on a side of a surface of the partition walls in a thickness direction of the partition walls,

the trapping layer does not contain a platinum group element,

the trapping layer is disposed on a side of a second surface positioned on an opposite side of a first surface of the partition walls having the platinum group element-containing catalyst layer disposed on the entire surface thereof, and

the trapping layer is disposed in a range of at least up to 50% with respect to an overall length of the honeycomb structure starting from the outflow end face, and is not disposed in a range of at least up to 40% with respect to the overall length starting from the inflow end face, in an extending direction of the cells of the honeycomb structure.

[2] The honeycomb filter according to the above [1], wherein a porosity of the platinum group element-containing catalyst layer is from 50 to 90%.

[3] The honeycomb filter according to the above [1] or [2], wherein a thickness of the platinum group element-containing catalyst layer is from 10 to 40 jAm.

[4] The honeycomb filter according to any one of the above [1] to [3], wherein the platinum group element-containing catalyst layer is a catalyst layer containing an oxide of at least one element selected from the group consisting of aluminum, zirconium, and cerium.

[5] The honeycomb filter according to any one of the above [1] to [4], wherein a porosity of the trapping layer is from 70 to 90%.

[6] The honeycomb filter according to any one of the above [1] or [5], wherein a thickness of the trapping layer is from 10 to 50 ptm.

[7] The honeycomb filter according to any one of the above [1] to [6], wherein the trapping layer is a layer containing an oxide of at least one element selected from the group consisting of aluminum, magnesium, zirconium, cerium, and silicon.

[8] The honeycomb filter according to any one of the above [1] to [7], the platinum group element-containing catalyst layer is disposed on at least a part of the side of a second surface of the partition walls in a range of up to 60% with respect to the overall length starting from the inflow end face in the extending direction of the cells of the honeycomb structure.

The honeycomb filter according to the present invention increases the trapping performance for trapping the PM contained in exhaust gas and the purification performance for purifying the harmful components contained in the exhaust gas.

That is, in the honeycomb filter according to the present invention, the “platinum group element-containing catalyst layer” is disposed on the entire surface of the side of the inner surface of the partition walls surrounding either one of the inflow cells or the outflow cells, and the contact of the platinum group element-containing catalyst layer with the exhaust gas is promoted, such that the purification performance can be effectively improved. In addition, the partition walls on the side of the outflow end face in which a large amount of gas flows are selectively provided with the “trapping layer” for trapping the particulate matter in the exhaust gas, such that the trapping performance can effectively be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view schematically showing a first embodiment of a honeycomb filter of the present invention;

FIG. 2 is a plane view of a side of an inflow end face of the honeycomb filter shown in FIG. 1;

FIG. 3 is a plane view of a side of an outflow end face of the honeycomb filter shown in FIG. 1;

FIG. 4 is a cross-sectional view schematically showing a cross section taken along line A-A′ of FIG. 2;

FIG. 5 is a perspective view schematically showing a second embodiment of a honeycomb filter of the present invention;

FIG. 6 is a plane view of a side of an inflow end face of the honeycomb filter shown in FIG. 5;

FIG. 7 is a plane view of a side of an outflow end face of the honeycomb filter shown in FIG. 5; and

FIG. 8 is a cross-sectional view schematically showing a cross section taken along line B-B′ of FIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to the following embodiments. Therefore, it should be understood that the following embodiments appropriately modified, improved, and the like based on the ordinary knowledge of a person skilled in the art without deviating from the purpose of the invention fall within the scope of the present invention.

(1) Honeycomb Filter (First Embodiment)

A first embodiment of a honeycomb filter of the present invention is a honeycomb filter 100 as shown in FIGS. 1 to 4. Here, FIG. 1 is a perspective view schematically showing the first embodiment of the honeycomb filter of the present invention. FIG. 2 is a plane view of a side of an inflow end face of the honeycomb filter shown in FIG. 1. FIG. 3 is a plane view of a side of an outflow end face of the honeycomb filter shown in FIG. 1. FIG. 4 is a cross-sectional view schematically showing a cross section taken along line A-A′ of FIG. 2.

As shown in FIGS. 1 to 4, the honeycomb filter 100 according to the present embodiment includes a honeycomb structure 4 and plugging portions 5. The honeycomb structure 4 has porous partition walls 1 disposed so as to surround a plurality of cells 2 as through channels of fluid extending from an inflow end face 11 to an outflow end face 12. The honeycomb structure 4 shown in FIGS. 1 to 4 is formed in a round pillar shape having the inflow end face 11 and the outflow end face 12 as both end faces, and further has a circumferential wall 3 on a circumferential side face thereof. That is, the circumferential wall 3 is disposed so as to surround the partition walls 1 disposed in a shape of lattice.

The plugging portions 5 are disposed so as to plug either one of end portions of each of the cells 2 on the side of the inflow end face 11 or the side of the outflow end face 12. Hereinafter, among a plurality of cells 2, the cells 2 in which the plugging portions 5 are arranged in an end portion on the side of the outflow end face 12 and which are opened on the side of the inflow end face 11 are defined as “inflow cells 2 a”. In addition, among the plurality of cells 2, the cells in which the plugging portions 5 are arranged in end portion on the side of the inflow end face 11 and which are opened on the side of the outflow end face 12 are defined as “outflow cells 2 b”. In the honeycomb filter 100 according to the present embodiment, it is preferable that the inflow cell 2 a and the outflow cell 2 b are alternately disposed via the partition walls 1.

In the honeycomb filter 100, the honeycomb structure 4 is configured as follows. That is, the honeycomb structure 4 further includes a platinum group element-containing catalyst layer 14 and a trapping layer 15. The platinum group element-containing catalyst layer 14 and the trapping layer 15 are porous layers disposed on the surface of the partition walls 1. The platinum group element-containing catalyst layer 14 is constituted by an exhaust gas purifying catalyst containing a platinum group element. The trapping layer 15 is for trapping particulate matter (PM) in the exhaust gas. It should be noted that the trapping layer 15 does not contain the platinum group element and can be distinguished from the platinum group element-containing catalyst layer 14 depending on the presence or absence of the platinum group element.

The platinum group element-containing catalyst layer 14 is disposed at least on an entire surface of a side of an inner surface of the partition walls 1 surrounding either one of the inflow cells 2 a or the outflow cells 2 b. In addition, the platinum group element-containing catalyst layer 14 is present only on a side of a surface of the partition walls 1 in a thickness direction of the partition walls 1. For example, in the honeycomb filter 100 shown in FIGS. 1 to 4, the platinum group element-containing catalyst layer 14 is disposed on the entire surface of the inner surface of the partition walls 1 surrounding the inflow cells 2 a. When the platinum group element-containing catalyst layer 14 is disposed at least on the entire surface of the inner surface of the partition walls 1 which surround either one of the inflow cells 2 a and the outflow cells 2 b, the platinum group element-containing catalyst layer 14 may be disposed on a surface of the partition walls 1 other than the above-mentioned range. However, it is preferable that the platinum group element-containing catalyst layer 14 is not disposed in a range in which the trapping layer 15 is disposed. The “present only on the side of the surface of the partition walls 1” means that in a thickness direction of the partition walls 1, the platinum group element-containing catalyst is not present between 0.1T and 1.0T starting from a position of the above 0.1T (where, T indicates the thickness of the partition walls 1) in the thickness direction of the partition walls 1 starting from the inner surface of the partition walls 1.

The trapping layer 15 is disposed on a side of a second surface positioned on an opposite side of a first surface of the partition walls 1 having the platinum group element-containing catalyst layer 14 disposed on the entire surface thereof. Here, the first surface of the partition walls 1 is “the inner surface of the partition walls 1 surrounding the inflow cells 2 a” in the honeycomb filter 100 shown in FIGS. 1 to 4, and the second surface of the partition walls 1 is “the inner surface of the partition walls 1 surrounding the outflow cells 2 b”.

Furthermore, the trapping layer 15 is disposed in a range of at least up to 50% with respect to the overall length of the honeycomb structure 4 starting from the outflow end face 12 and is not disposed in a range of at least up to 40% with respect to the overall length starting from the inflow end face 11, in an extending direction of the cells 2 of the honeycomb structure 4.

Here, the platinum group element includes ruthenium, rhodium, palladium, osmium, iridium, and platinum. Hereinafter, the platinum group element may be referred to as “PGM”. The platinum group element-containing catalyst layer 14 is a catalyst coating layer which is formed by coating the platinum group element-containing catalyst on a predetermined surface of the partition walls 1.

The honeycomb filter 100 has the platinum group element-containing catalyst layer 14 and the trapping layer 15 configured as described above, and has an effect of making trapping performance for trapping the PM contained in the exhaust gas excellent and purification performance for purifying harmful components contained the exhaust gas excellent.

That is, in the honeycomb filter 100, the “platinum group element-containing catalyst layer 14” is disposed on the entire surface of an inner surface of the partition walls 1 which surrounds either one of an inflow cells 2 a or an outflow cells 2 b. Therefore, the contact of the platinum group element-containing catalyst layer 14 with the exhaust gas is promoted, such that the purification performance can be effectively improved. In addition, the partition walls 1 on the side of the outflow end face 12 in which a large amount of gas flows is selectively provided with the “trapping layer 15” for trapping the PM in the exhaust gas, such that the trapping performance can effectively be improved.

The porosity of the platinum group element-containing catalyst layer 14 is preferably from 50 to 90%, more preferably from 60 to 80%, and particularly preferably from 60 to 70%. If the porosity of the platinum group element-containing catalyst layer 14 is less than 50%, the pressure loss may be increased. On the other hand, if the porosity of the platinum group element-containing catalyst layer 14 exceeds 90%, the trapping efficiency may deteriorate.

An average pore diameter of the platinum group element-containing catalyst layer 14 is preferably from 1 to 7 μm, more preferably from 1 to 5 μm, and particularly preferably from 1 to 3 μm.

The porosity and the average pore diameter of the platinum group element-containing catalyst layer 14 can be measured by the following method. First, a cross-sectional portion of the platinum group element-containing catalyst layer 14 is observed by a scanning electron microscope (hereinafter, also referred to as “SEM”) to acquire the SEM image thereof. The SEM image is magnified to 200 times for observation. The SEM an abbreviation for “scanning electron microscope”. As the scanning electron microscope, for example, a scanning electron microscope “model number: S3200-N” manufactured by Hitachi High-Technologies Corporation can be used. Next, a substantial portion of the platinum group element-containing catalyst layer 14 and a void portion in the platinum group element-containing catalyst layer 14 are binarized by analyzing the acquired SEM image. Then, a percentage of a ratio of the void portion in the platinum group element-containing catalyst layer 14 with respect to a total area of the substantial portion and the void portion of the platinum group element-containing catalyst layer 14 is calculated, and the calculated value is set to be the porosity of the platinum group element-containing catalyst layer 14. In addition, separately, a hollow wall between the respective particle diameters in the SEM image is binarized, and a size of the hollow wall is directly measured with a scale, and the pore diameter of the platinum group element-containing catalyst layer 14 is calculated. An average value of the calculated pore diameters is set to be an average pore diameter of the platinum group element-containing catalyst layer 14.

In addition, the platinum group element-containing catalyst layer 14 is preferably a catalyst layer constituted by a platinum group element-containing catalyst having a particle diameter of 1 to 10 μm. With such a configuration, the platinum group element-containing catalyst layer 14 can effectively be disposed on the inner surface of the partition walls 1 surrounding the inflow cells 2 a.

The thickness of the platinum group element-containing catalyst layer 14 is preferably from 10 to 40 m, more preferably from 20 to 35 μm, and particularly preferably from 20 to 30 μm. If the thickness of the platinum group element-containing catalyst layer 14 is less than 10 μm, it is not preferable because the contact of the platinum group element-containing catalyst layer 14 with gas is lowered. On the other hand, if the thickness of the platinum group element-containing catalyst layer 14 exceeds 40 μm, it is not preferable because the pressure loss may be increased.

The thickness of the platinum group element-containing catalyst layer 14 can be measured by the following method. First, a cross-sectional portion of the platinum group element-containing catalyst layer 14 is observed by a scanning electron microscope to acquire the SEM image thereof. Next, from the acquired SEM image, the thickness of the platinum group element-containing catalyst layer 14 is directly measured using the scale.

It is preferable that the platinum group element-containing catalyst layer 14 is a catalyst layer containing an oxide of at least one element selected from the group consisting of aluminum, zirconium, and cerium. It is preferable that the catalyst layer containing such an oxide contains 1 to 3% by mass of a platinum group element with respect to the total mass of the catalyst layer. A composition of the platinum group element-containing catalyst layer 14 can be measured by, for example, fluorescent X-ray analysis (XRF; X-ray fluorescence). Specifically, the composition analysis of the platinum group element-containing catalyst layer 14 can be performed by detecting fluorescent X-rays intrinsic to each element generated by irradiating a sample with X-rays.

The trapping layer 15 does not contain the platinum group element and is preferably a layer containing an oxide of at least one element selected from the group consisting of aluminum, magnesium, zirconium, cerium, and silicon, for example. In the present specification, the “no containing platinum group element” means that the content of the platinum group element in the component is 0.1% by mass or less. Therefore, the trapping layer 15 may contain 0.1% by mass or less of the platinum group element with respect to a total mass of the trapping layer 15. As compared with the platinum group element-containing catalyst layer 14, such a trapping layer 15 can be prepared from a less expensive material and can further improve the trapping performance while reducing the manufacturing cost of the honeycomb filter 100. The composition of the trapping layer 15 can be measured by the same method as the platinum group element-containing catalyst layer 14, for example, by fluorescent X-ray analysis (XRF).

When the composition of the trapping layer 15 is analyzed by the above-described fluorescent X-ray analysis, it is preferable that the content of the platinum group element is 0.1% by mass or less, and more preferably, is equal to or less than a detection limit of an analysis device.

The porosity of the trapping layer 15 is preferably from 70 to 90%, more preferably from 75 to 90%, and particularly preferably from 80 to 90%. If the porosity of the trapping layer 15 is less than 70%, it is not preferable because the pressure loss may be increased. On the other hand, if the porosity of the trapping layer 15 exceeds 90%, it is not preferable because the trapping efficiency may deteriorate.

An average pore diameter of the trapping layer 15 is preferably from 1 to 7 μm, more preferably from 1 to 5 μm, and particularly preferably from 1 to 3 μm. The porosity and the average pore diameter of the trapping layer 15 can be measured by the same method as the method for measuring the porosity and the average pore diameter of the platinum group element-containing catalyst layer 14 described above.

In addition, the trapping layer 15 is preferably made of particles of an oxide containing at least one element selected from the group consisting of aluminum, magnesium, zirconium, cerium, and silicon having a particle diameter of 1 to 10 μm. With such a configuration, the PM in the exhaust gas can be effectively trapped by the trapping layer 15 disposed on the surface of the partition walls 1.

The thickness of the trapping layer 15 is preferably from 10 to 40 μm, more preferably from 20 to 35 μm, and particularly preferably from 20 to 30 μm. If the thickness of the trapping layer 15 is less than 10 μm, it is not preferable because the improvement amount of the trapping efficiency may be lowered. On the other hand, if the thickness of the trapping layer 15 exceeds 40 μm, it is not preferable because the pressure loss may be increased. The thickness of the trapping layer 15 can be measured by the same method as the method for measuring the thickness of the platinum group element-containing catalyst layer 14 described above.

The trapping layer 15 is disposed in at least “a range of up to 50% with respect to the overall length of the honeycomb structure 4 starting from the outflow end face 12 in an extending direction of the cells 2 of the honeycomb structure 4” and is not disposed in a range of at least up to 40% with respect to the overall length starting from the inflow end face 11. The range in which the trapping layer 15 is disposed is preferably in the range of 50% with respect to the overall length starting from the outflow end face 12, and more preferably in the range of 40% with respect to the overall length starting from the outflow end face 12. With such a configuration, the increase in the pressure loss of the honeycomb filter 100 can be effectively suppressed and the trapping efficiency can be favorably improved. Hereinafter, the “overall length in the extending direction of the cells 2 of the honeycomb structure 4” may be referred to as “the overall length of the honeycomb structure 4” or simply “the overall length”.

In the honeycomb filter 100 shown in FIGS. 1 to 4, the platinum group element-containing catalyst layer 14 is disposed in an entire area of the cross section, in the cross section orthogonal to the extending direction of the cells 2 of the honeycomb structure 4. However, the platinum group element-containing catalyst layer 14 may be disposed on the side of the inner surface of the partition walls 1 surrounding at least either one of the inflow cells 2 a or the outflow cells 2 b in the cross section. For example, although not shown in the drawings, the platinum group element-containing catalyst layer may be disposed only on the partition walls present in a central portion of the honeycomb structure. Here, in a cross section orthogonal to the extending direction of the cells of the honeycomb structure, “the central portion” is defined as a range within 60% of a length from a center of gravity of the cross section to a circumferential edge of the cross section. With such a configuration, it is possible to suppress the increase in the pressure loss while favorably maintaining the purification performance and effectively reduce the manufacturing cost. Of course, in the honeycomb filter 100 shown in FIGS. 1 to 4, a form in which, in a cross section orthogonal to the extending direction of the cells 2 of the honeycomb structure 4, the platinum group element-containing catalyst layer 14 is disposed in the entire area of the cross section, is one preferable embodiment.

The porosity of the partition walls 1 of the honeycomb structure 4 is preferably from 50 to 70%, more preferably from 55 to 65%, and particularly preferably from 60 to 65%. The porosity of the partition walls 1 is a value measured by a mercury porosimetry. The porosity of the partition walls 1 can be measured using, for example, AutoPore 9500 (product name) manufactured by Micromeritics Instrument Corp. If the porosity of the partition walls 1 is less than 50%, it is not preferable because a permeation resistance of the partition walls 1 is increased and the pressure loss is increased. If the porosity of the partition walls 1 exceeds 70%, it is not preferable because the strength is remarkably decreased.

The average pore diameter of the partition walls 1 is preferably from 10 to 25 μm, more preferably from 10 to 20 μm, and particularly preferably from 15 to 20 μm. The average pore diameter of the partition walls 1 is a value measured by the mercury porosimetry. The average pore diameter of the partition walls 1 can be measured using, for example, AutoPore 9500 (product name) manufactured by Micromeritics Instrument Corp. If the average pore diameter of the partition walls 1 is less than 10 μm, it is not preferable because a permeation resistance of the partition walls 1 is increased and the pressure loss is increased. If the average pore diameter of the partition walls 1 exceeds 25 μm, it is not preferable because the gas flow concentrates on the portion of the large pore and therefore the trapping efficiency deteriorates.

In the honeycomb structure 4, the thickness of the partition walls 1 is preferably from 0.15 to 0.38 mm, more preferably from 0.18 to 0.33 mm, and particularly preferably from 0.20 to 0.31 mm. The thickness of the partition walls 1 can be measured using, for example, the scanning electron microscope or the microscope. If the thickness of the partition walls 1 is less than 0.15 mm, the sufficient strength may not be obtained. On the other hand, if the thickness of the partition walls 1 exceeds 0.38 mm, the pressure loss may be increased when the catalyst is loaded onto the partition walls 1.

The shape of the cells 2 formed in the honeycomb structure 4 is not particularly limited. For example, in the cross section orthogonal to the extending direction of the cells 2, the shape of the cells 2 may be a polygon, a circle, an ellipse, or the like. Examples of the polygon can include a triangle, a quadrangle, a pentagon, a hexagon, an octagon and the like. The shape of the cells 2 is preferably a triangle, a quadrangle, a pentagon, a hexagon, or an octagon. As for the shape of the cells 2, all the cells 2 may have the same shape, or the cells 2 may have different shapes each other. For example, although not shown, quadrangular cells and octagonal cells may be mixed. As for the size of the cells 2, all the cells 2 may have the same size, or the cells 2 may have different sizes each other. For example, although not shown, of a plurality of cells, some cells may have a larger size, and the other cells may have a smaller size. In the present invention, the cell means a space surrounded by the partition wall.

A cell density of the cells 2 defined by the partition walls 1 is preferably from 31 to 54 cells/cm², and more preferably from 39 to 47 cells/cm². With such a configuration, the honeycomb filter can be suitably used as a filter for trapping PM in exhaust gas emitted from an engine of an automobile or the like.

The circumferential wall 3 of the honeycomb structure 4 may be formed integrally with the partition walls 1 or may be a circumferential coating layer formed by coating a circumferential coating material so as to surround the partition walls 1. Although not shown, at the time of the manufacturing, the circumferential coating layer may be provided on the circumferential side of the partition walls after the partition walls are integrally formed with the circumferential wall and then the formed circumferential wall is removed by the known methods such as grinding processing.

The shape of the honeycomb structure 4 is not particularly limited. The shape of the honeycomb structure 4 may include a pillar shape in which the inflow end face 11 and the outflow end face 12 include a circular shape, an elliptical shape, and a polygonal shape.

The size of the honeycomb structure 4, for example, the length in the extending direction of the cells 2 of the honeycomb structure 4 (hereinafter, also referred to as “overall length”) or the size of the cross section (hereinafter also referred to as “cross-sectional area”) orthogonal to the extending direction of the cells 2 of the honeycomb structure 4 is not particularly limited. Each size may be appropriately selected so as to obtain the optimum purification performance at the time of using the honeycomb filter 100. The overall length of the honeycomb structure 4 is preferably from 90 to 160 mm, and more preferably from 120 to 140 mm. In addition, the cross-sectional area of the honeycomb structure 4 is preferably from 8000 to 16000 mm², and more preferably from 10000 to 14000 mm².

The material of the partition walls 1 preferably is at least one selected from the group consisting of cordierite, silicon carbide, a silicon-silicon carbide based composite material, mullite, alumina, aluminum titanate, silicon nitride, and silicon carbide-cordierite based composite material. The material constituting the partition walls 1 is preferably a material containing 30% by mass or more, more preferably a material containing 40% by mass or more, and particularly preferably a material including 50% by mass or more of the materials listed in the above group. In the honeycomb filter 100 of the present embodiment, the material constituting the partition walls 1 is particularly preferably cordierite.

(2) Honeycomb Filter (Second Embodiment)

Next, a second embodiment of a honeycomb filter according to the present invention will be described below. The second embodiment of a honeycomb filter according to the present invention is a honeycomb filter 200 as shown in FIGS. 5 to 8. Here, FIG. 5 is a perspective view schematically showing a second embodiment of a honeycomb filter according to the present invention. FIG. 6 is a plane view of a side of an inflow end face of the honeycomb filter shown in FIG. 5. FIG. 7 is a plane view of a side of an outflow end face of the honeycomb filter shown in FIG. 5. FIG. 8 is a cross-sectional view schematically showing a cross section taken along line B-B′ of FIG. 6.

As shown in FIGS. 5 to 8, a honeycomb filter 200 according to the present embodiment includes a honeycomb structure 24 and plugging portions 25. The honeycomb structure 24 has porous partition walls 21 disposed so as to surround a plurality of cells 22 as through channels of fluid extending from an inflow end face 31 to an outflow end face 32. The honeycomb structure 24 is formed in a round pillar shape having an inflow end face 31 and an outflow end face 32 as both end faces, and further has a circumferential wall 23 on a circumferential side face thereof.

The plugging portions 25 are disposed so as to plug either one of end portions of each of the cells 22 on the side of the inflow end face 31 or the side of the outflow end face 32. The cells 22 in which the plugging portions 25 are arranged in an end portion on the side of the outflow end face 32 are defined as “inflow cells 22 a”, the cells 22 in which the plugging portions 25 are arranged in end portion on the side of the inflow end face 31 are defined as “outflow cells 22 b”.

Even in the honeycomb filter 200, the honeycomb structure 24 further includes a platinum group element-containing catalyst layer 34 and a trapping layer 35. However, in the honeycomb filter 200 according to the present embodiment, the platinum group element-containing catalyst layer 34 is disposed on an entire surface of a side of an inner surface of a partition walls 21 surrounding an outflow cells 22 b.

In addition, the trapping layer 35 is disposed on a side of a second surface positioned on an opposite side of a first surface (that is, an inner surface of the partition walls 21 surrounding the outflow cells 22 b) of the partition walls 21 having the platinum group element-containing catalyst layer 34 disposed over the entire surface thereof. That is, the trapping layer 35 is disposed on the side of the inner surface of the partition walls 21 surrounding the inflow cells 22 a. The trapping layer 35 is disposed in a range of up to 50% with respect to the overall length of the honeycomb structure 24 from the outflow end face 32 and is not disposed in a range of up to 40% with respect to the overall length starting from the inflow end face 31, in the extending direction of the cells 22 of the honeycomb structure 24.

Furthermore, in the honeycomb filter 200 according to the embodiment, the platinum group element-containing catalyst layer 34 is formed on the entire surface of the side of the inner surface of the partition walls 21 surrounding the outflow cells 22 b, and furthermore is also disposed on the side of the second surface of the partition walls 21 on which the trapping layer 35 is disposed. That is, the platinum group element-containing catalyst layer 34 is also disposed on the inner surface of the partition walls 21 surrounding the inflow cells 22 a. With such a configuration, the contact of the platinum group element-containing catalyst layer 34 with the exhaust gas is further promoted, such that the purification performance can be effectively improved.

As in the honeycomb filter 200 according to the embodiment, when the platinum group element-containing catalyst layer 34 is also disposed on the side of the second surface of the partition walls 21, the platinum group element-containing catalyst layer 34 is preferably disposed in a part of a range of up to 60% with respect to the overall length of the honeycomb structure 24 starting from the inflow end face 31. In particular, the platinum group element-containing catalyst layer 34 is already disposed on the side of the entire surface of the first surface of the partition walls 21 and the trapping layer 35 is disposed in the range of up to 50% with respect to the overall length starting from the outflow end face 32 of the side of the second surface of the partition walls 21. Therefore, the platinum group element-containing catalyst layer 34 is not disposed on the entire surface of the partition walls 21, but as described above, is disposed at a part of the side of the second surface of the partition walls 21 near the side of the inflow end face 31 to be able to obtain the purification performance necessary and sufficient for purifying the exhaust gas.

(3) Method for Manufacturing Honeycomb Filter

The method for manufacturing a honeycomb filter according to the present invention is not particularly limited, and for example, the following method can be mentioned.

First, a plastic kneaded material for manufacturing partition walls of a honeycomb structure is prepared. The kneaded material for manufacturing the partition walls of the honeycomb structure can be prepared by appropriately adding additives such as a binder, a pore former, and water to a raw material powder for preparing a suitable material for the above-mentioned partition walls. Examples of the raw material powder may include powder of alumina, talc, kaolin, and silica. Examples of the binder may include methylcellulose, hydroxypropyl methylcellulose or the like. In addition, examples of the additive may include a surfactant

Next, a pillar honeycomb formed body having partition walls for defining a plurality of cells and a circumferential wall disposed to surround the partition walls is prepared by extruding the thus obtained kneaded material. Next, the obtained honeycomb formed body is dried with microwave and hot air, for example.

Next, the plugging portions are formed on the dried honeycomb formed body. The method for forming plugging portions can be performed according to the conventionally known method for manufacturing a honeycomb filter. For example, first, a mask is applied to the inflow end face of the honeycomb formed body so that the inflow cell is covered. Thereafter, the end portion having the mask of the honeycomb formed body is immersed in a plugging slurry, and an open end having no mask of the outflow cell is filled with the plugging slurry. Thereafter, even for the outflow end face of the honeycomb formed body, the open end of the inflow cell is filled with the plugging slurry by the same method as described above. Thereafter, the honeycomb formed body having the plugging portions formed therein is further dried with a hot air dryer.

Next, the honeycomb formed body having the plugging portions formed therein is fired to prepare a honeycomb filter precursor prior to disposing the platinum group element-containing catalyst layer. A firing temperature and a firing atmosphere at the time of firing the honeycomb formed body differ depending on a raw material for preparing the honeycomb formed body, and those skilled in the art can select a firing temperature and a firing atmosphere most suitable for the selected material.

Next, the platinum group element-containing catalyst for preparing the platinum group element-containing catalyst layer is prepared. As the platinum group element-containing catalyst, for example, a catalyst in which a platinum group element is loaded onto an aluminum oxide having a particle diameter of 1 to 10 μm can be used. Such aluminum oxide is applied to the entire surface of the side of the inner surface of the partition walls surrounding either one of the inflow cells or the outflow cells of the honeycomb filter precursor. In addition, when the aluminum oxide is further applied to the surface (second surface) positioned on the side opposite to the entire surface to which the aluminum oxide is applied, the aluminum oxide is applied with zone coat. As a specific method of the zone coat, for example, the following methods can be mentioned. First, a catalyst layer forming slurry containing catalyst powder such as aluminum oxide loading the platinum group element and an appropriate solvent (for example, ion exchanged water) and a dispersing agent is prepared. Next, the catalyst layer forming slurry is poured from the inflow end face or the outflow end face of the honeycomb filter precursor and sucked from the outflow end face or the inflow end face to apply the platinum group element-containing catalyst to the surface of the partition walls surrounding predetermined cells. The method for applying the platinum group element-containing catalyst to the entire surface of the inner surface of the partition walls can also be carried out by the same method as the above-mentioned zone coat. Thereafter, the platinum group element-containing catalyst applied to the surface of the partition walls is fired at 500° C. to prepare the platinum group element-containing catalyst layer. By adjusting at least one of the viscosity of the catalyst layer forming slurry and the pressure at the time of suction, the platinum group element-containing catalyst layer is disposed only on the surface of the partition walls. In addition, as a method of the zone coat, a platinum group element-containing catalyst layer can also be applied to the surface of the partition walls by dipping the catalyst layer forming slurry.

Next, the trapping layer is formed in a range of at least up to 50% with respect to the overall length starting from the outflow end face of the honeycomb structure by the following method. First, a material for the trapping layer for preparing the trapping layer is prepared. As a raw material for the trapping layer, for example, a silicon oxide having a particle diameter of 1 to 10 μm can be used. Such a silicon oxide is applied with the zone coat in a range of up to 50% with respect to the overall length starting from at least the outflow end face of the honeycomb structure. Thereafter, the zone-coated platinum group element-containing catalyst is fired at 500° C. to prepare the trapping layer. By doing so, the honeycomb filter according to the present invention can be manufactured.

EXAMPLES

Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not limited to these examples.

Example 1

First, alumina, talc, kaolin, and silica raw materials for preparing partition walls of a honeycomb structure were prepared. 2 parts by mass of a dispersing medium and 7 parts by mass of an organic binder were each added to the prepared alumina, talc, kaolin, and silica raw materials (total 100 parts by mass), mixed, and kneaded to prepare kneaded material. As the dispersing medium, water was used. As the organic binder, methyl cellulose was used. As the dispersing agent, a surfactant was used.

Next, the kneaded material was extruded using a die for preparing a honeycomb formed body to obtain a honeycomb formed body whose overall shape is a round pillar shape. The shape of the cell of the honeycomb formed body was a rectangle.

Next, after the honeycomb formed body was dried by a microwave dryer and completely dried by a hot air dryer again, both end faces of the honeycomb formed body were cut and adjusted to predetermined dimensions.

Next, plugging portions were formed on the dried honeycomb formed body. Specifically, first, a mask was applied to the inflow end face of the honeycomb formed body so that the inflow cell is covered. Thereafter, the end portion having the mask of the honeycomb formed body is immersed in a plugging slurry, and an open end having no mask of the outflow cell is filled with the plugging slurry. Thereafter, even for the outflow end face of the honeycomb formed body, the open end of the inflow cell is filled with the plugging slurry by the same method as described above. Thereafter, the honeycomb formed body having the plugging portions formed therein is further dried with a hot air dryer.

Next, the dried honeycomb formed body was degreased and fired to prepare a honeycomb filter precursor prior to disposing the platinum group element-containing catalyst layer.

Next, by the following method, the platinum group element-containing catalyst layer was prepared on the entire surface of the side of the inner surface of the partition walls surrounding the inflow cells of the honeycomb filter precursor, and in the range of up to 50% with respect to the overall length starting from the inflow end face of the side of the inner surface of the partition walls surrounding the outflow cells. First, a catalyst layer forming slurry containing a powder of aluminum oxide onto which palladium as a platinum group element is loaded, ion exchanged water, and a dispersing agent was prepared. Next, the catalyst layer forming slurry was poured from the inflow end face of the honeycomb filter precursor, and the poured catalyst layer forming slurry was sucked from the outflow end face while adjusting a pressure at the time of suction so that the platinum element-containing catalyst layer is applied only to the inner surface of the partition walls. By doing so, the platinum group element-containing catalyst was applied to the surface of the partition walls surrounding the inflow cells of the honeycomb filter precursor.

Next, by the following method, the trapping layer was prepared in the range of up to 50% with respect to the overall length starting from the outflow end face of the side of the inner surface of the partition walls surrounding the outflow cells of the honeycomb filter precursor. First, the trapping layer forming slurry containing a powder of silicon oxide, ion exchanged water, and a dispersing agent as raw materials for the trapping layer was prepared. Next, the trapping layer forming slurry was poured from the outflow end face of the honeycomb filter precursor and sucked from the inflow end face to apply the trapping layer to the surface of the partition walls surrounding the outflow cell of the honeycomb filter precursor.

The shape of the honeycomb filter of Example 1 was a round pillar shape in which the shape of the inflow end face and the outflow end face was a circle. In addition, a length in an extending direction of the cells of the honeycomb filter was 127 mm. A diameter of the end face of the honeycomb filter was 118 mm. In the honeycomb structure constituting the honeycomb filter, a thickness of the partition wall was 0.22 mm, and a cell density was 47 cells/cm². A porosity of the partition walls of the honeycomb structure was 63%.

The platinum group element-containing catalyst layer was disposed only on the entire surface of the side of the inner surface of the partition walls surrounding the inflow cells, and only in the range of 50% with respect to the overall length starting from the inflow end face of the side of the inner surface of the partition walls surrounding the outflow cells. A porosity of the platinum group element-containing catalyst layer was 65%. The thickness of the platinum group element-containing catalyst layer was 30 jim.

The trapping layer was disposed in the range of up to 50% with respect to the overall length starting from the outflow end face of the side of the inner surface of the partition walls surrounding the outflow cells. A porosity of the trapping layer was 80%. The thickness of the trapping layer was 30 μm.

TABLE 1 Disposition range of platinum group element-containing catalyst Disposition spot of Partition wall Partition wall platinum group surrounding surrounding element-containing Disposition range inflow cell outflow cell catalyst of trapping layer Comparative — — Entire surface in pore of — — Example 1 partition wall Comparative — — Entire surface in pore of — — Example 2 partition wall and surface of partition wall of 50% from outflow end face Example 1 Entire surface 50% from Surface of partition wall Outflow 50% from outflow end face cell outflow end face Example 2 Entire surface Absence Surface of partition wall Outflow 50% from cell outflow end face Example 3 Entire surface 60% from Surface of partition wall Outflow 40% from outflow end face cell outflow end face Example 4 50% from Entire surface Surface of partition wall Inflow 50% from inflow end face cell outflow end face Example 5 60% from Entire surface Surface of partition wall Inflow 60% from inflow end face cell outflow end face Example 6 Absence Entire surface Surface of partition wall Inflow 40% from cell outflow end face

With respect to the honeycomb filter of Example 1, “trapping efficiency performance”, “pressure loss performance”, and “exhaust gas purification performance” were evaluated by the following method. The results are shown in Table 2.

(Trapping Efficiency Performance)

First, an exhaust gas purifying apparatus using a honeycomb filter of each example as an exhaust gas purifying filter was manufactured. The manufactured exhaust gas purification apparatus was connected to a side of an outlet of an engine exhaust manifold of a 1.2 L direct injection type gasoline engine vehicle to measure the number of soot contained in the gas emitted from the outlet of the exhaust gas purification apparatus by a PN measurement method. The “PN measurement method” is a measurement method proposed by a particle measurement program (abbreviated as PMP) at the Conference of Exhaust Gas Energy Experts (abbreviated as GRPE) of Automobile Standard Harmonized World Forum (abbreviated as WP29) in the European Economic Committee (abbreviated as ECE) of the United Nations (abbreviated as UN). Specifically, in the determination of the number of soot, the cumulative number of soot emitted after traveling in a Worldwide harmonized Light duty Test Cycle (WLTC) mode is defined as the number of soot of the exhaust gas purifying apparatus to be determined, and the trapping efficiency is measured. The trapping efficiency performance was evaluated on the basis of the following evaluation criteria which were based on the measured values of each trapping efficiency. When the values of the trapping efficiency of the exhaust gas purifying apparatus using the honeycomb filter of Comparative Example 1 was set to be 100% in a column of “trapping efficiency ratio (%)” in the following Table 2, the values (%) of the trapping efficiency of the exhaust gas purifying apparatus using the honeycomb filters of each example were shown.

Evaluation “excellent”: When the value of the trapping efficiency of the exhaust gas purifying apparatus using the honeycomb filter of Comparative Example 1 was set to be 100% and when the value of the trapping efficiency of the exhaust gas purifying apparatus using the honeycomb filter to be evaluated was 160% or more, the evaluation was “excellent”.

Evaluation “good”: When the value of the trapping efficiency of the exhaust gas purifying apparatus using the honeycomb filter of Comparative Example 1 was set to be 100% and when the value of the trapping efficiency of the exhaust gas purifying apparatus using the honeycomb filter to be evaluated exceeded 155% and was less than 160%, the evaluation was “good”.

Evaluation “permission”: When the value of the trapping efficiency of the exhaust gas purifying apparatus using the honeycomb filter of Comparative Example 1 was set to be 100% and when the value of the trapping efficiency of the exhaust gas purifying apparatus using the honeycomb filter to be evaluated exceeded 100% and was 155% or less, the evaluation was “permission”.

(Pressure Loss Performance)

Exhaust gas emitted from a 1.4 L direct injection type gasoline engine was introduced into honeycomb filters of each example and soot in the exhaust gas was trapped in partition walls of the honeycomb filter. The soot was trapped until the amount of soot deposited per unit volume (1 L) of the honeycomb filter reached 1 g/L. Then, the engine exhaust gas of 200° C. was flowed in at a flow rate of 1.0 Nm³/min in a state in which the deposited amount of soot was 1 g/L, and the pressure between the side of the inflow end face and the side of the outflow end face of the honeycomb filter was measured. Then, a pressure loss (kPa) of the honeycomb filter was obtained by calculating a pressure difference between the side of the inflow end face and the side of the outflow end face. The pressure loss performance was evaluated on the basis of the following evaluation criteria which were based on the measured values of each pressure loss. In the column of “pressure loss ratio (%)” in the following Table 2, values (%) of the pressure loss of the honeycomb filters of each example were shown when the value of the pressure loss of the honeycomb filter of Comparative Example 1 was 100%.

Evaluation “excellent”: When the value of the pressure loss of the honeycomb filter of Comparative Example 1 was set to be 100% and when the value of the pressure loss of the honeycomb filter to be evaluated was 85% or less, the evaluation was “excellent”.

Evaluation “good”: When the value of the pressure loss of the honeycomb filter of Comparative Example 1 was set to be 100% and when the value of the pressure loss of the honeycomb filter to be evaluated exceeded 85% and was 90% or less, the evaluation was “good”.

Evaluation “permission”: When the value of the pressure loss of the honeycomb filter of Comparative Example 1 was set to be 100% and when the value of the pressure loss of the honeycomb filter to be evaluated exceeded 90% and was less than 100%, the evaluation was “permission”.

Evaluation “no permission”: When the value of the pressure loss of the honeycomb filter of Comparative Example 1 was set to be 100% and when the value of the pressure loss of the honeycomb filter to be evaluated was 100% or more, the evaluation was “no permission”.

(Exhaust Gas Purification Performance)

First, an exhaust gas purifying apparatus using a honeycomb filter of each example as an exhaust gas purifying filter was manufactured. The manufactured exhaust gas purification apparatus was connected to a side of an outlet of an engine exhaust manifold of a 1.2 L direct injection type gasoline engine vehicle to measure a concentration of NOx contained in the gas emitted from the outlet of the exhaust gas purification apparatus and obtain a purification efficiency of NOx. The exhaust gas purification performance was evaluated on the basis of the following evaluation criteria which were based on the measured values of the purification efficiency of each NOx. When the values of the purification efficiency of NOx of the exhaust gas purifying apparatus using the honeycomb filter of Comparative Example 1 was set to be 100% in a column of “purification efficiency ratio (%) of NOx” in the following Table 2, the values (%) of the purification efficiency of NOx of the exhaust gas purifying apparatus using the honeycomb filters of each example were shown.

Evaluation “excellent”: When the value of the purification efficiency of NOx of the exhaust gas purifying apparatus using the honeycomb filter of Comparative Example 1 was set to be 100% and when the value of the purification efficiency of NOx of the exhaust gas purifying apparatus using the honeycomb filter to be evaluated was 160% or more, the evaluation was “excellent”.

Evaluation “good”: When the value of the purification efficiency of NOx of the exhaust gas purifying apparatus using the honeycomb filter of Comparative Example 1 was set to be 100% and when the value of the purification efficiency of NOx of the exhaust gas purifying apparatus using the honeycomb filter to be evaluated was 150% or more and less than 160%, the evaluation was “good”.

Evaluation “permission”: When the value of the purification efficiency of NOx of the exhaust gas purifying apparatus using the honeycomb filter of Comparative Example 1 was set to be 100% and when the value of the purification efficiency of NOx of the exhaust gas purifying apparatus using the honeycomb filter to be evaluated exceeded 100% and was less than 150%, the evaluation was “permission”.

TABLE 2 Purification Purification Trapping efficiency Trapping Pressure efficiency of efficiency Pressure ratio of Trapping Exhaust gas efficiency loss NOx ratio loss ratio NOx efficiency Pressure loss purification (%) (kPa) (%) (%) (%) (%) performance performance performance Comparative 60.00 3.000 60 100% 100% 100% Criterion Criterion Criterion Example 1 Comparative 66.25 3.400 65 110% 113% 108% Permission No permission Permission Example 2 Example 1 95.00 2.800 95 158% 93% 158% Good Permission Good Example 2 91.25 2.425 90 152% 81% 150% Permission Excellent Good Example 3 92.50 2.650 97 154% 88% 162% Permission Good Excellent Example 4 95.00 2.650 90 158% 88% 150% Good Good Good Example 5 97.50 2.800 91 163% 93% 151% Excellent Permission Good Example 6 95.00 2.650 80 158% 88% 133% Good Good Permission

Examples 2 to 6

Honeycomb filters were prepared in the same method as Example 1 except that a disposition range and a disposition spot of a platinum group element-containing catalyst and a trapping layer were changed as shown in the above Table 1. With respect to the honeycomb filters of Examples 2 to 6, “trapping efficiency performance”, “pressure loss performance”, and “exhaust gas purification performance” were evaluated by the same method as Example 1. The results are shown in Table 2.

Comparative Examples 1 and 2

Honeycomb filters were prepared in the same method as Example 1 except that a disposition range and a disposition spot of a platinum group element-containing catalyst and a trapping layer were changed as shown in the above Table 1. In Comparative Example 1, as in the following methods, the platinum group element-containing catalyst was loaded into the inside of pores (onto the entire surface of inside of pores) of partition walls of a honeycomb structure. First, a catalyst layer forming slurry containing a catalyst powder of aluminum oxide onto which a platinum group element is loaded, ion exchanged water, and a dispersing agent was prepared. Next, the platinum group element-containing catalyst was applied to the entire surface of partition walls of a honeycomb filter precursor by impregnating the catalyst layer forming slurry from an inflow end face and an outflow end face of the honeycomb filter precursor. Thereafter, the platinum group element-containing catalyst was fired at 500° C. to prepare the platinum group element-containing catalyst layer. In Comparative Example 2, the platinum group element-containing catalyst was also applied to the surface of the partition walls in the range of 50% with respect to the overall length starting from the outflow end face in addition to the inside of the pores of the partition walls, thereby preparing the platinum group element-containing catalyst layer.

(Result)

It was confirmed that the honeycomb filters of Examples 1 to 6 exceeded the respective performances of the honeycomb filter of Comparative Example 1 serving as a reference, in all evaluations of the “trapping efficiency performance”, the “pressure loss performance”, and the “exhaust gas purification performance”. Therefore, it has been found that the honeycomb filters of Examples 1 to 6 are also excellent in the trapping performance and the purification performance, and can suppress the increase in the pressure loss as compared with the conventional honeycomb filter. On the other hand, as compared with the honeycomb filters of Examples 1 to 6, the honeycomb filter of Comparative Example 2 was inferior in all evaluation results of “trapping efficiency performance”, “pressure loss performance”, and “exhaust gas purification performance”.

The honeycomb filter according to the present invention can be used as the filter for trapping the particulate matter in the exhaust gas.

DESCRIPTION OF REFERENCE NUMERALS

1 and 21: partition wall, 2 and 22: cell, 2 a and 22 a: inflow cell, 2 b and 22 b: outflow cell, 3 and 23: circumferential wall, 4 and 24: honeycomb structure, 5 and 25: plugging portion, 11 and 31: inflow end face, 12 and 32: outflow end face, 14 and 34: platinum group element-containing catalyst layer, 15 and 35: trapping layer, and 100 and 200: honeycomb filter. 

What is claimed is:
 1. A honeycomb filter, comprising: a honeycomb structure which has porous partition walls disposed so as to surround a plurality of cells as through channels of fluid extending from an inflow end face to an outflow end face; and plugging portions disposed to plug either one of end portions of each of the cells on the side of the inflow end face or the side of the outflow end face, wherein the cells in which the plugging portions are arranged in the end portion on the side of the outflow end face and which are opened on the side of the inflow end face are defined as inflow cells, the cells in which the plugging portions are arranged in the end portion on the side of the inflow end face and which are opened on the side of the outflow end face are defined as outflow cells, the honeycomb structure further includes a platinum group element-containing catalyst layer constituted by an exhaust gas purifying catalyst containing a platinum group element and a trapping layer which traps particulate matter in exhaust gas, the platinum group element-containing catalyst layer is disposed at least on an entire surface of a side of an inner surface of the partition walls surrounding either one of the inflow cells or the outflow cells, the platinum group element-containing catalyst layer is present only on a side of a surface of the partition walls in a thickness direction of the partition walls, the trapping layer does not contain a platinum group element, the trapping layer is disposed on a side of a second surface positioned on an opposite side of a first surface of the partition walls having the platinum group element-containing catalyst layer disposed on the entire surface thereof, and the trapping layer is disposed in a range of at least up to 50% with respect to an overall length of the honeycomb structure starting from the outflow end face and is not disposed in a range of at least up to 40% with respect to the overall length starting from the inflow end face, in an extending direction of the cells of the honeycomb structure.
 2. The honeycomb filter according to claim 1, wherein a porosity of the platinum group element-containing catalyst layer is from 50 to 90%.
 3. The honeycomb filter according to claim 1, wherein a thickness of the platinum group element-containing catalyst layer is from 10 to 40 μm.
 4. The honeycomb filter according to claim 1, wherein the platinum group element-containing catalyst layer is a catalyst layer containing an oxide of at least one element selected from the group consisting of aluminum, zirconium, and cerium.
 5. The honeycomb filter according to claim 1, wherein a porosity of the trapping layer is from 70 to 90%.
 6. The honeycomb filter according to claim 1, wherein a thickness of the trapping layer is from 10 to 50 μm.
 7. The honeycomb filter according to claim 1, wherein the trapping layer is a layer containing an oxide of at least one element selected from the group consisting of aluminum, magnesium, zirconium, cerium, and silicon.
 8. The honeycomb filter according to claim 1, wherein the platinum group element-containing catalyst layer is disposed on at least a part of the side of a second surface of the partition walls in a range of up to 60% with respect to the overall length starting from the inflow end face in the extending direction of the cells of the honeycomb structure. 